1. Field of the Invention
The present invention relates generally to a zoom lens system and more specifically but not exclusively to a zoom lens system used in photographic optical systems.
2. Description of the Related Art
Recently, digital still cameras including those with a solid-state imaging device, such as a charged-couple device (CCD) sensor, for still image photography have been attracting attention. A photographic optical system for digital still cameras can have a significantly short overall lens length is in demand in order to reduce the size of the camera. Such an optical system requires a wide angle due to the characteristics of a still image.
A conventional zoom lens system used as a photographing optical system of a reduced-size digital still camera is a negative-leading lens system in which a lens unit can have a negative refractive power precedes the other lens units. An example of a negative-leading lens system is a zoom lens system including a lens unit can have a negative refractive power disposed closest to the object and one to three lens units can have a positive refractive power disposed on the image side of the lens unit can have a negative refractive power. Examples of such a zoom lens system are disclosed in Japanese Patent Publication Nos. 6-66008 (corresponding to U.S. Pat. No. 4,662,723) and 7-52256 (corresponding to U.S. Pat. No. 4,733,952), Japanese Patent Laid-Open Nos. 60-31110, 10-104520, and 11-23967 (corresponding to U.S. Pat. No. 6,124,984), and U.S. Pat. No. 5,434,710.
A popular conventional photographic optical system included in a digital still camera has lens units arranged such that a lens unit can have positive refractive power is disposed closest to the image to obtain telecentric optical characteristics, which reduces shading caused by the solid-state imaging device. However, due to recent advancements in technology, conventional solid-state imaging devices now can cause less shading even with a lens system can have a short eye-relief. The lens system included in such a solid-state imaging device does not have to have a positive refractive power lens unit can have disposed closest to the image and thus may contribute to producing a lens system can have a short overall length.
A negative-leading lens system in which a lens unit can have a negative refractive power is disposed closest to the image, may include three lens units can have negative, positive, and negative refractive powers, disposed in the respective order from the object side. Such a lens system is disclosed in Japanese Patent Laid-Open Nos. 2-63007 (corresponding to U.S. Pat. No. 4,955,700), 7-77655, 8-179209, 11-211985, 9-113809 (corresponding to U.S. Pat. No. 5,793,534), and 8-320435 and U.S. Pat. No. 4,936,661.
However, the zoom lens systems discussed (excluding Japanese Patent Laid-Open No. 8-320435) are for photographic optical systems for film cameras, and thus for these systems the eye-point is extremely close to the image surface. Hence, even if a solid-state imaging device has excellent shading characteristics, the angle of the off-axis rays entering the solid-state imaging devices becomes sharp and is thus unsuitable for a photographing optical system for digital still cameras. Each zoom lens system disclosed above contains a relatively large number of lenses and thus the size of the lens system cannot be reduced sufficiently enough.
The zoom lens disclosed in Japanese Patent Laid-Open No. 8-320435 is capable of forming an image on a solid-state imaging device. However, the negative refractive power of the third lens unit is small. Therefore, when a solid-state imaging device capable of can have a short eye-relief is taken into consideration, there is still leeway for reducing the overall size of the lens system.